Crystal Structure of Rb4C60 and K4C60 under Pressure

We show that Rb_4C_60 transforms from its ambient pressure orientationally disordered tetragonal structure to an ordered orthorhombic structure (isostructural to Cs_4C_60) at or above 0.4 GPa. This opens the possibility of studying the previously observed metal-insulator transition under pressure in this narrow band system as a function of a continuous variable, rather than formulating distinct models for conducting and insulating fullerides. No such transition is observed in K_4C_60 to 2.0 GPa